This article is from the Toys FAQ, by firstname.lastname@example.org (Scott Gordon) with numerous contributions by others.
You can buy
them just about anywhere.
Believe it or not, there are two U.S. Agent figures:
U.S. Agent I - Part of the last assortment of the late Marvel
Super-Heroes line. He was a repaint of Captain America,
available in wide release and can still be had in many stores for
$5 or less.
U.S. Agent II - If you look on the back an Iron Man series III
card, you will see a U.S. Agent figure, but don't bother looking
on the racks, he won't be there. He was canceled, but not before
93 samples were made. Tomarts Toy Digest received one of these
samples and had a U.S. Agent Giveaway Contest. An rtm reader was
the lucky winner of this fully-functional fully-painted shield-
launching figure in a baggy (see Tomarts #25). Ted Sallis, Toy
Biz toy sculptor, received one as well. I assume some lucky
people with good connections will get the rest. Ted Sallis says
a new U.S. Agent figure may be released some time in the future,
but it will be retooled. So if you ever wondered just what a
"rare" figure really is, this is it.